Core insert



FIG. 1 is a top view of the core insert;

FIG. 2 is a top perspective view of the core insert;

FIG. 3 is a rear view of the core insert;

FIG. 4 is a left side view of the core insert; and,

FIG. 5 is a front view of the core insert. 

The ornamental design for a core insert, as shown and described. 